Coupon Off Box

Hi, I want a box created in my checkout pages where a contest winner can enter a number an proceed through checkout without being charged. Example: Say they win $40. They come the the store to buy anything they want up to $40 without being charged anything at all, ie shipping or taxes. They add "whatever" to their cart and if it's under $40 ie $25 the cart lets them through without charging them anything, and sends them a email informing them they still have a $15 credit. If they spend over $40 then they proceed through checkout and pay for the remainder of their purchase ...less $40. It just calculates there total (counting shipping and taxes) and minuses off $40. Presently, my cart (squirrelcart) has a coupon field box but doesn't really work correctly in this application, but it might be customized. In the control panel there is a place where I can set the coupon settings to either percent off or dollar amount off, and choose the coupon redeem "number". But if I set it to say $40 the customer has to purchase at lest $40 or they are not allowed through checkout. Also it doesn't calculate shipping as part of that $40. Shipping is real time UPS and USPS based on weight. It's calculated in step four in the checkout process. As far as I know the cart uses all php programming. I guess that's it. The biggest problem I see is the shipping. Thanks